Polymers and other plastics have been developed which are useful in many applications including automotive parts, engineering plastics, toys, containers, molded items and packaging films. It is often desirable to decorate or to protect items made from these plastics by applying a coating or an aesthetically pleasing design to the plastic. It is obviously important that the coating is tightly bonded to the plastic surface. Plastic surfaces as well as certain non-plastic surfaces such as metal including aluminum and galvanized steel are not particularly receptive to the application of adherent coatings because the surfaces are essentially non-porous. Accordingly, there is a continuing need for coating compositions which exhibit excellent adhesion to plastics and metals, and in particular, to plastics.
Improved adhesion of decorative and protective coating compositions to various substrates, and in particular, non-porous substrates, has been achieved by the application of base coats or primer coats to the substrate prior to the application of the desired decorative coating. Since primer coatings are not visible, they do not have to possess all of the desirable characteristics of the top coatings such as color retention, mar-resistance, chalk-resistance, dirt-resistance, etc. Thus, primer coatings can be formulated with major emphasis on adhesion to substrates, and adhesion to subsequent basecoats and/or topcoats.
For many years, curable coating compositions useful as paints, varnishes, etc., have been dissolved in volatile hydrocarbon solvents to facilitate the deposition of thin films on the substrates to be coated and to enable the solvent to evaporate into the atmosphere within a reasonable period of time leaving a dry non-tacky coating. The use of such volatile hydrocarbon solvents as diluents, therefore, results in air pollution problems.
More recently, the coatings industry has directed its attention to the problem of volatile organic emissions from organic coating compositions. These efforts have been encouraged by various governmental and state agencies concerned with the air pollution caused by the use of volatile hydrocarbon solvents. Such efforts by the coatings industry has resulted in the development of a number of high-solids resin coating formulations which contains significantly reduced amounts of solvents, and, in some instances, little or no solvent.
Some difficulty has been observed in coating certain substrates with these high solids, low solvent coating composition which generally contain significant amounts of water. Primer coatings deposited on non-porous substrates such as plastic and metals generally exhibit poor adhesion, particularly when topcoats are applied over the primer coating.